Electric machines in current prior art are related to the process known as “electromechanical energy conversion”, therefore, an electric machine and the link between an electric system and a mechanical system.
Conversion is reversible in these machines, i.e., if the conversion is from mechanical to electrical the machine is called a generator, on the other hand, if the conversion is from electrical to mechanical the machine is called a motor, reason why this type of machine can be operated as a generator or motor, being called AC machines if the system is Alternating Current, and DC machines if Direct Current (generators or motors).
Within both categories mentioned, there are a variety of motors and generators, including those that use permanent magnets to produce magnetic fluxes.
Permanent magnets are used in the stator and coils in the rotor, or magnets in the rotor and coils in the stator in an electric machine (motor or generator), this being to economize space, since a permanent magnet occupies less space than an electromagnet having an iron core and copper wire coil.
Magnets are usually employed in motors or generators, segmented for reasons of said space economy. Permanent magnets are also used to raise the efficiency of motors, since the permanent magnets have no coil, copper loss (I2R) is reduced.
Generally, a problem found in these motors that count on permanent magnets is that the magnetic field flux created by the magnet is static and permanent, in addition to the fact that this flux cannot be controlled from a minimum value to a maximum value or vice-versa, as is the case with electromagnets that produce fully controllable fluxes because they respond to the current and the number of coil windings.
In conventional motors that use permanent magnets, the lower current corresponds to a lower magnetic flux and a higher current corresponds to a higher magnetic flux.
Therefore, and for instance, it is impossible to create a series motor when the stator is made of permanent magnets and the rotor of is made of coil; the same goes for the inverse case.
In many types of series motors (rotor coils in series with stator coils), of variable speed that operate with electromagnets only, the amperage is reduced on lowering the voltage, for this reason, the motor speed is proportionally reduced, including, proportionally, the back emf. Inversely, on raising the voltage, the amperage is increased and therefore the speed, including the back emf, is proportionally increased.
In motors with permanent magnets, located in the stator or rotor coils, or vice-versa, the back emf is proportional to the permanent magnet flux and is not controllable.
In view of this problem, it is not recommended that permanent magnets be placed in variable speed motors, since when the current is cut from the motor, the rotor, by inertia, continues to turn due to the magnet's fixed field present, producing back emf, which in most cases is prejudicial.